The present invention relates generally to the configuration of optical component assemblies, and in particular to systems and methods for limit sensing, gap maintenance and gap measurement between surfaces such as in polished optical surfaces used in fiber collimators.
Optical fiber has become increasingly important in many applications involving the transmission of light. Light transmitted through a fiber is subjected to different types of optical interactions to filter, modulate, split, combine or otherwise act on the light. In many cases one or more fibers are led into an enclosure operating as an optical system. The input light entering the enclosure, usually but not always on one fiber, interacts with some optical device within the enclosure, and the resulting light exits the enclosure via one or more fibers. An example of such a two-port system is an optical isolator in which two polarizers sandwiching a Faraday rotator are positioned between two fibers that have collimating lenses adjacent their free ends. The polarization angles are set such that light can propagate in one direction through the isolator but is prevented from propagating in the opposite direction. Another example is a dielectric interference filter that transmits or reflects selected wavelengths.
In practice, the fibers used in such optical systems are typically held in collimator assemblies that are easily aligned to the enclosure of the optical system. A commercial collimator includes as its most fundamental components the fiber, a small glass tube (also referred to as a capillary) that holds the exposed fiber end, and a graded index (GRIN) lens. A GRIN type lens used with optical fiber is generally a cylindrical piece of optical glass with a length longer than its diameter. It is fabricated to have a radially varying index of refraction that is greater towards the center, with the result being that it produces a focusing effect similar to a convex lens. The fiber is held in the collimator assembly by a cylindrical ferrule (this ferrule fiber holder is further referred as a glass tube). The distance (i.e. gap) between the fiber end and the GRIN lens is critical for collimation. Once this distance has been set, the fiber and the GRIN lens are fixed together, for example by epoxy or by laser welding, to maintain the desired collimation. The fixed displacement between the fiber end and the GRIN lens should provide an optically well-characterized beam, and will result in minimum insertion loss of the optical system.
In the typical process of assembling a collimator, the GRIN lens is bonded to the glass tube (i.e., the holder of the fiber) with its angled (or faceted) side, while its plano side is facing outward (toward the next intermediate optical component). The fiber is received and held in constant position within the glass tube along its central axis, and the tube-fiber assembly is used for further alignment. Thereafter, the tube""s positions (and hence the fiber""s position) is varied or adjusted in respect to the GRIN lens position until some optimum condition is achieved. At that point, the tube is bonded to the GRIN lens. Gap sensing (or limit sensing) between the fiber end (i.e., tube end) and the GRIN lens is an important step in the manufacturing and calibration of optical components such as collimators discussed above.
Conventionally, the assembly and alignment (involving gap or limit sensing) of optical components have involved a long, tedious and labour-intensive operation. Conventional techniques include the use of cameras and complex image processing techniques, or ultrasound and strain gauge schemes to find xe2x80x9cnear touchxe2x80x9d conditions. Further, other conventional systems involve the use of optical interferometers or back-reflection methods to determine when surfaces are parallel and used to measure gap width between the two surfaces.
In accordance with one aspect of the present invention there is provided a method for controlling a relative movement between faces of first and second optical components, one end face of the first optical component being faced with one end face of the second optical component, the method comprising the steps of: providing first attaching means, the first optical component being attached to the first attaching means; providing second attaching means, the second optical component being attached to the second attaching means, the first and second optical components attached to the respective attaching means being capable of relatively moving with a gap being defined between the two faces in a direction; providing an electromotive force (EMF) to cause the two optical components to relatively move in the direction, so that the faces of the two optical components are in proximity; detecting contact between portions of the faces of the two optical components; and separating the faces of the two optical components a predetermined amount.
In an exemplary embodiment, the step of detecting contact further includes the step of detecting a change to the vibration of the movable arm when the faces of the two optical components touch each other. For example, the step of detecting a change can include the following steps: obtaining a first AC voltage in response to the AC oscillation voltage; obtaining a second AC voltage induced from the current flowing in the coil; and processing the first and second AC voltages with reference to their electrical parameters to detect contact of portions of the faces of the two optical components.
In a further exemplary embodiment, the method includes the step of digitally detecting the resonant frequency. For example, the step of digitally detecting can include the following steps: providing a first digital data; providing an analog signal to the coil with reference to the first digital data, a current flowing in the coil in response to the analog signal; obtaining an induced analog voltage resulting from the flowing of the current in the coil; converting the induced analog voltage to a second digital data; and processing the first and second digital data to detect the resonant frequency with reference to the first and second digital data.
In a further exemplary embodiment, the step of providing a first data comprises the step of choosing the frequency of an sinusoidal input to the coil, the first digital data containing information of the chosen frequency; the step of converting comprises the step of digitizing the induced analog voltage, the second digital data containing frequency information of a digitized voltage; and the step of the processing comprises the steps of: (i) comparing the chosen frequency information of the first digital data and the frequency information of the second digital data; and (ii) in a case where a predetermined condition with the both frequencies is met, providing a new first digital data containing information of a newly chosen frequency.
In accordance with another aspect of the present invention there is provided a system for controlling a relative movement between faces of first and second optical components, one end face of the first optical component being faced with one end face of the second optical component, the system comprising: a first attachment body for attaching the first optical component thereto; a second attachment body for attaching the second optical component thereto, the first and second optical components attached to the respective attachment bodies being capable of relatively moving with a gap being defined between the two faces in a direction; an electromotive force (EMF) device for providing an EMF to cause the two optical components to relatively move in the direction, so that the faces of the two optical components are in proximity; means for detecting contact between portions of the faces of the two optical components; and means for separating the faces of the two optical components a predetermined amount.
In an exemplary embodiment, the system further comprising means for providing an AC oscillation voltage to the coil, wherein an AC current flows in the coil, so that the EMF vibrates the movable arm mechanically, the movable arm having a mechanical resonance. Contact is detected at a frequency of the resonance of the vibrating movable arm, the resonant frequency being varied when the faces of the two optical components touch each other.
In a further exemplary embodiment, the change of the resonant frequency is detected in response to the AC oscillation voltage and an induced AC voltage resulted from the current flowing in the coil, wherein with reference to parameters of the AC oscillation voltage and the induced AC voltage, so that from the detection of the resonant frequency change, contact of portions of the faces of the two optical components is detected.
In a further exemplary embodiment, the system includes a stepping motor for controlling the movement of the movable arm, the movement of the movable arm being further controlled in response to the detection of contact of portions of the two faces of the two optical components.
In a further exemplary embodiment, the system includes means for (i) providing a first digital data; (ii) providing an analog signal to the coil with reference to the first digital data, a current flowing in the coil in response to the analog signal; (iii) obtaining an induced analog voltage resulting from the flowing of the current in the coil; (iv) converting the induced analog voltage to a second digital data; (v) processing the first and second digital data to detect the resonant frequency with reference to the first and second digital data, wherein the resonant frequency is detected. The first digital data contains information of the chosen frequency of an sinusoidal input to the coil; and the second digital data contains frequency information of a digitized voltage derived from the induced analog voltage, the chosen frequency information of the first digital data and the frequency information of the second digital data being compared, so that in a case where a predetermined condition with the both frequencies is met, a new first digital data containing information of a newly chosen frequency is provided to perform a new detection of the resonant frequency.
In summary, according to exemplary embodiments of the present invention, the following are features are provided by the systems and methods described:
(a) Limit sensing of two optical surfaces in a manufacturing system involving electrical systems by exploiting mechanical resonance. Fine tuning support is provided by dithering a movable arm or flexure assembly by sending a sinusoidal current to a coil in presence of a magnetic field from a permanent magnet. The two optical surfaces whose gap is to be maintained are mounted on the flexure and a static arm.
(b) All three axes can have flexure-based moving arms to sense the gap.
(c) Independent of the material filled in the gap (e.g., water, alcohol and glue). Limit indication occurs when a portion of the two surfaces touch. The limit indication is independent of relative position of flexure and micrometer position.
(d) After limit sensing, maintaining a particular predetermined gap (i.e., separation between the two surfaces) is simplified. By variable dithering the gap between any two optical surfaces of a given device, measurement of gap is also possible.
(e) Since the mechanical dithering before the two optical surfaces touch drops drastically after the two surfaces touch, there is effectively no damage done to the optical surfaces. Also, the method and apparatus of the present invention is scalablexe2x80x94one can maintain mechanical dither in the sub-micrometer range.
(f) To measure gap along any direction, it is sufficient to dither in one of the axes.
(g) A separate sensor is not required to measure or sense the gap. The coil in a magnetic field is used to move and sense the gap.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.